1. Field of the Invention
The present invention relates to a voltage level adjusting circuit and to a power supply circuit useful for a portable battery-powered electronic device and, more particularly, to a power supply circuit having a linear regulator for receiving a level adjusted voltage.
2. Related Art
Liner regulators are typically used for power conversion in portable electronic devices, such as portable computers and portable audio/video devices. A linear regulator accepts a power level from a power source, such as an alternating current/direct current (AC/DC) adapter or a battery, and conditions to provide regulated output voltages to system components at precise levels corresponding to the associated component biasing level. A variety of manufactures distribute these linear regulators. For example, National Semiconductor manufactures LM140A/LM140/LM340A/LM7800C series monolithic 3-terminal positive regulators, which employ internal current-limiting, thermal shutdown and safe-area compensation. These devices generally include advantageous features to protect the output voltages from short circuits by internal current limiter and thermal overload protection circuitry.
Considering output current characteristics of such linear regulators, even though a linear regulator intends to regulate its output voltage, its output current is inversely proportional to its input voltage and ambient temperature. In other words, stability of the linear regulator depends on its input voltage and temperature. Low input voltage and low temperature are thus necessary to acquire a stable output voltage of the linear regulator. In particular, regulating efficiency of the linear regulator is absolutely influenced by the difference between its input voltage and target output voltage.
Also, a portable computer exemplary of contemporary practice in the art is operated by either a battery or an AC/DC adapter. The battery is a rechargeable battery such as a Li-Ion battery, which is attachable to and detachable from the portable computer and generally provides a direct current (DC) voltage V.sub.bat of about 9-13 volts (V). The AC/DC adapter converts a commercial alternating current (AC) voltage of 110/220 volts into a DC voltage V.sub.adp of about 19 V. The AC/DC adapter is electrically coupled to an adapter connector of the portable computer.
Further, a power supply circuit exemplary of contemporary practice in the art for the portable computer includes a direct current/direct current (DC/DC) converter which is supplied with the DC voltages V.sub.bat and V.sub.adp through respective diodes. The DC/DC converter provides several DC voltages for individual circuit components of the portable computer. Specifically, the DC/DC converter includes two components, i.e., a system power supply unit and a linear regulator. The system power supply unit supplies various DC voltages for a system unit, including a central processing unit (CPU), other local controllers, several memories, etc., and for a direct current/alternating current (DC/AC) converter which supplies a liquid crystal device (LCD) panel with an AC voltage of about 500V for the backlighting of the panel.
The linear regulator provides a regulated voltage V.sub.mc to a microcontroller for system power management and the like. The microcontroller is presented with the regulated voltage V.sub.mc even during a system power-off mode where the power supplies of both the system unit and the DC/AC converter are interrupted. The linear regulator is applied with two different voltages (i. e, the battery-supplied voltage V.sub.bat of about 8-14 V and the adapter-supplied voltage V.sub.adp of about 18-20 V) and produces the regulated voltage V.sub.mc of 5V for the microcontroller, for example.
Generally, all components of a portable electronic device such as a portable computer can be designed from the portability view point. Thus, it is surely justifiable is that the linear regulator is designed for the battery-supplied voltage V.sub.bat rather than the adapter-supplied voltage V.sub.adp. However, since the linear regulator has to be commonly used for the two different input voltages V.sub.bat and V.sub.adp, its output voltage will vary with its input voltages, as discussed above. In particular, when the AC/DC adapter is used for the portable computer, the regulation efficiency of the linear regulator can be degraded since its input-output voltage difference is greater than that when the battery is used. Furthermore, when its temperature exceeds an acceptable limit because of the higher input voltage, it can stop supplying the power because of its inherent thermal protection. For these reasons, it can be difficult for the regulator to provide a stable output voltage when the portable computer is operated by the AC adapter.
U.S. Pat. No. 5,168,205 to Kan et al., entitled METHOD AND APPARATUS FOR CHARGING A BATTERY IN HIGH AMP AND AUTOMATIC CHARGING MODES, discloses a battery charger adapted to be electrically connected to the terminals of a battery for providing a charge to the battery, the battery charger comprising a device for supplying charging current to the battery, the current supplying device being operable to alternatively and selectively supply current at a first and second rate, the first rate providing for high rate charging of the battery, circuitry for sensing the voltage of the battery while the charger is connected to the battery, the sensing circuitry being adapted to be coupled to the battery, circuitry for alternatively selecting the first rate and the second rate, the selecting circuitry being coupled to the supplying circuitry and to the severing circuitry, circuitry for severing the supply of charging current to the battery when the second rate is selected, and when the sensing circuitry senses battery voltage in excess of a first predetermined threshold voltage, and for resuming the supply of current to the battery at the second rate when the sensing circuitry senses battery voltage below a second predetermined threshold voltage lower than the first predetermined threshold voltage, the severing circuitry being coupled to the current supplying device and to the sensing circuitry.
U.S. Pat. No. 5,411,816 to Patino, entitled METHOD AND APPARATUS FOR DETERMINING BATTERY CHARACTERISTICS, discloses a battery charging system including a battery and charger. It is disclosed that the battery includes a thermistor for determining the temperature of the battery and a resistor for informing the charger of the current capacity of the battery, and the charger includes a transistor for effectively switching between a measuring thermistor and a resistor at a charger input terminal.
U.S. Pat. No. 5,477,123 to Allen et al., entitled CONNECTION MULTIPLE BATTERIES TO BATTERY POWERED DEVICES, discloses a circuit for serially connecting multiple batteries to a battery-powered device such as a notebook computer or camcorder so that the device will serially charge or discharge the batteries. The device is disclosed to include at least two controllable switch circuits for selectively connecting batteries to the battery-powered device, a power flow sensor for sensing power flow from the device to the selected battery or vice versa, and a selector circuit for sequentially enabling the switch circuits to sequentially connect the batteries to the batter-powered device so that the batteries will sequentially charge or discharge.
U.S. Pat. No. 5,055,763 to Johnson et al., entitled ELECTRONIC BATTERY CHARGER DEVICE AND METHOD, discloses a battery charger device and method including a charging circuit having a source of charging energy and connections for installing one or more batteries to be charged, the device having a controllable switch associated with each battery to be charged and a microprocessor for controlling the switches to establish a charging sequence of distinct repeated charging periods for each battery, a display device associated with each battery under control of a microprocessor for individually indicating the charging status of each battery, the microprocessor having inputs at which various parameters with respect to the batteries being charged are applied such as battery voltages, ambient temperatures and charging currents, the microprocessor being programmed to terminate a voltage charging operation with respect to any one or more of the batteries if the measured difference between the charging and non charging voltage of any one or more of the batteries during succeeding charging periods exceeds some predetermined value, or if the charging current, or the ambient temperature of the battery being charged is outside of a predetermined range. It is disclosed that the present device and method can also include a back up termination means which responds to measurements of the battery voltage during succeeding charging periods and determines therefrom that on a predetermined number of succeeding cycles the condition of the battery voltage has changed by at least some predetermined amount.
U.S. Pat. No. 5,561,361 to Sengupta et al., entitled COMPUTER POWER SUPPLY AND BATTERY RECHARGING SYSTEM, discloses a power supply system for use with a portable personal computer, which includes a smart battery pack and a charging system. It is disclosed that the smart battery pack is provided with a dedicated microcontroller for controlling the charging level of the battery charger system. In particular, the status of the battery including the voltage and temperature of the battery is applied to the microcontroller along with a signal representative of the current load demand of the computer system. The microcontroller, in turn, is disclosed to provide a control signal in the form of fixed frequency, variable duty cycle pulse width modulated (PWM) signal for controlling the charging level of the battery charger system. It is disclosed that the duty cycle of the PWM signal is used to regulate the charging current supplied by the battery charger. In particular, the DC value of the PWM signal is used as a reference to control the charging current of the regulator to provide a variable output charging current with a relatively wide current range. As such, the battery charger is adapted to efficiently utilize the residual capacity of the battery charger system for optimizing charging of the battery packs during all operating conditions of the computer system. It is also disclosed that the use of a PWM signal from the battery pack to control the battery charger enables a single type of battery charger to be utilized for various battery technologies.
U.S. Pat. No. 5,598,041 to Willis, entitled EFFICIENT FAULT TOLERANT SWITCHING CIRCUIT FOR REDUNDANT D.C. POWER SUPPLIES, discloses two power sources which are connected to a load by a pair of MOSFET control switches, each connected respectively in series between one of the sources and the load, with their integral diodes forwardly biased between the power source and load. It is disclosed that an isolating MOSFET switch is connected in series between the pair of MOSFET control switches and the load with its integral diode back biased between the power sources and the load. Two control lines are disclosed to control the on-off state of all three MOSFET switches so that the isolating MOSFET switch is on when either control switch is on and is off when both control switches are off.
U.S. Pat. No. 5,621,301 to Allen, entitled CONNECTING MULTIPLE BATTERIES TO BATTERY POWERED DEVICES, discloses a circuit for serially connecting multiple batteries to a battery-powered device such as a notebook computer or camcorder so that the device will serially charge or discharge the batteries. The device is disclosed to include at least two controllable switch circuits for selectively connecting batteries to the battery-powered device, a power flow sensor for sensing power flow from the device to the selected battery or vice versa, and a selector circuit for sequentially enabling the switch circuits to sequentially connect the batteries to the battery-powered device so that the batteries will sequentially charge or discharge.
U.S. Pat. No. 5,629,602 to Makino, entitled PORTABLE ELECTRONIC EQUIPMENT WITH ATTACHMENT FOR SUPPLYING POWER AND CHARGING BATTERY, discloses an electronic equipment which includes a container unit for storing a battery or an AC adaptor which are detachable from the container unit and exchangeable with each other, an input port unit for receiving electric power from an external source, and a connection attachment unit for storing the battery and the AC adaptor which are detachable from the connection attachment unit, so that the AC adaptor can be stored in the connection attachment unit to supply DC power to the electronic equipment through the input port in order to charge the battery stored in the container unit, so that another battery can be stored in the connection attachment unit to supply DC power to the electronic equipment through the input port in order to serve as an expansion battery in addition to the battery stored in the container unit, or so that both the battery and the AC adaptor can be stored in the connection attachment unit to charge the battery.
U.S. Pat. No. 5,637,981 to Nagai et al., entitled METHOD FOR CHARGING A SECONDARY BATTERY AND CHARGER USED THEREFOR USING CONSTANT CURRENT AND CONSTANT VOLTAGE, discloses a secondary battery charging method and a charger used therefor, in which the method uses a constant current and a constant voltage such that the secondary battery is first charged with the constant current until the terminal voltage of the battery becomes a reference voltage higher than the full charging voltage for the battery and then further charged with the constant voltage which is equal to the full charging voltage. These two charging operations are switched by using various kinds of detection and control circuits.
U.S. Pat. No. 5,646,503 to Stockstad, entitled METHOD FOR BALANCING POWER SOURCES AND STRUCTURE THEREFOR, discloses a power source balancing circuit which balances two power sources such as two battery cells. When the power source balancing circuit is enabled, it is disclosed that it compares a current flowing through the first battery cell and a first resistor with a current flowing through the second battery cell and a second resistor. It is disclosed because the resistance of the first resistor is equal to that of the second resistor, a difference between the two currents indicates a differences between the voltages of the two battery cells. If a current difference larger than a predetermined limit is detected, the battery cell with a higher voltage is discharged through a corresponding discharge resistor by switching on a corresponding switch. The corresponding switch is controlled by a corresponding flip-flop.
U.S. Pat. No. 5,694,028 to Salmonson et al., entitled METHOD AND APPARATUS FOR ADJUSTING THE POWER SUPPLY VOLTAGE PROVIDED TO A MICROPROCESSOR, discloses a method and apparatus for adjusting power supplied to a device when the device has a first and a second power input. It is disclosed that a first voltage level and a ground potential are provided and a second voltage level is created as a function of the first voltage level. The second voltage level is then buffered with a power transistor and, if the second voltage level is needed for a particular device, the buffered second voltage level is selectively applied to the device. It is also disclosed that the circuit is disabled when the second voltage supply is not needed.
U.S. Pat. No. 5,815,382 to Saint-Pierre et al., entitled TRACKING CIRCUIT FOR POWER SUPPLY OUTPUT CONTROL, discloses a tracking circuit for a power supply which includes an inverter, a su m g network, and a driver circuit. The inverter is disclosed to invert a feedback voltage, which is a variable load voltage level, about an inverter reference voltage level to provide an inverter output voltage level. It is disclosed that the inverter output voltage level is summed or averaged with the output voltage of the power supply with respect to the voltage of the return line to provide a summing voltage. It is also disclosed that the driver circuit signals the primary control circuit to adjust the power supply output voltage level so that the summing voltage is held constant and equal with a driver reference voltage level.
U.S. Pat. No. 5,867,007 to Kim, entitled SELECTION CIRCUIT FOR DUAL BATTERIES IN A BATTERY POWERED ELECTRONIC DEVICE, discloses a battery selection circuit for dual battery packs which discharges the batteries in a stable way without regard to voltage differences. It is disclosed if one of the batteries is installed, the battery is effectively selected to prolong battery life. When this circuit is adopted in a notebook computer, it is disclosed that battery usage time is substantially increased. The battery selection circuit of Kim 007 includes a battery detector for sensing the existence of the first and second battery packs and for producing the detect signals corresponding thereto, and these detect signals are provided to a microcontroller to produce first and second battery discharge enable signals. In response to the first and second battery discharge enable signals, first and second electronic switches respectively connect each power supply line from the first and second batteries to a power supply circuit of the device. Reverse current blocking diodes are provided in series with the first and second switches, and a low resistive current path circuit is provided in parallel with the reverse current blocking diodes. It is disclosed that the first switch and second switch as well as the low resistive current path circuit can include MOSFET transistors. A control circuit including logic gates is provided for opening the low resistive current path circuit when both the first and second batteries are detected, and for closing the corresponding low restive current path circuit when one of the first and second batteries is detected.